1. Field of the Invention
The present invention relates to an eyepiece device and a binocular.
2. Related Background Art
Eyepiece devices of optical apparatuses such as binoculars are provided with an eye cup for the purpose of preventing entrance of exterior light or making it easy to keep the positional relationship between the eyepiece device and the eyes constant. Some eyepiece devices having an eye cup are equipped with a mechanism for allowing the eye cup to move along the optical axis relative to the eyepiece barrel.
A known mechanism for allowing movement of the eye cup along the optical axis direction is provided with a twist-up ring that is movable along the optical axis direction relative to the eyepiece barrel, a twist-up adjusting pin provided on the eyepiece barrel and three pin engagement portions provided on the twist-up ring for retaining the twist-up pin (see Japanese Utility Model No. 3090513). With this mechanism, the eye cup can be retained steadily at three positions with respect to the optical axis direction.
When a person wearing eyeglasses uses an eyepiece device that allows adjustment of the position of the eye cup, the eye cup will be used in the state in which it is pushed (or retracted) into the eyepiece barrel (see FIGS. 1 and 2).
Letting D be the distance from the surface of the eyepiece lens G1 that constitutes the last lens in the optical system to the surface of the eyeglass lens G4 (or G5), t be the thickness of the eyeglass lens G4 (or G5), S1 be the distance from the backside surface of the eyeglass lens G4 (or G5) to the surface of the cornea E, S2 be the distance from the surface of the cornea E to the iris I, and X be the distance from the surface of the eyepiece lens G1 that constitutes the last lens in the optical system to the iris I, the equation X=D+t+S1+S2 holds.
The distance from the surface of the eyepiece lens G1 that constitutes the last lens in the optical system to the surface of the eyeglass lens G5 for farsightedness is generally smaller than the distance from the surface of the eyepiece lens G1 that constitutes the last lens in the optical system to the surface of the eyeglass lens G4 for nearsightedness. In addition, the thickness t of the eyeglass lens G5 for farsightedness is larger than the thickness t of the eyeglass lens G4 for nearsightedness.
Generally, the thickness t of the eyeglass lens G4 (or G5) is larger than 1.1 mm, the distance S1 from the backside surface of the eyeglass lens G4 (or G5) to the surface of the cornea E is larger than 12 mm, and the distance S2 from the surface of the cornea E to the iris I is approximately 3 mm (equivalent value in the air), and therefore in typical cases, the distance X from the surface of the eyepiece lens G1 that constitutes the last lens in the optical system to the iris I is larger than 20 mm.
The distance of the position of the eye point (i.e. the position at which a pupil is formed) EP of optical apparatuses such as binoculars from the most eye side surface of the eyepiece lens (that is, the surface of the eyepiece lens which surface is closest to the person using an optical apparatus.) is generally smaller than 20 mm. Accordingly, even when the eye cup is set to the fully retracted (or pushed in) state, the position of the eye point EP and the position of the iris of a person wearing eyeglasses do not coincide with each other. This means that eyeglass wearers cannot adjust the position of the eye cup to allow the position of their iris I to coincide with the eye point, and they cannot see whole the field of view.
On the other hand, when a person who is not wearing eyeglasses (i.e. a person who observes with naked eye) uses an eyepiece device that allows adjustment of the position of the eye cup, the eye cup will be used in the state in which it is extended (or pulled out) from the eyepiece barrel (see FIGS. 3 and 4).
Each one has his own face shape and there are differences in the shape of the faces. Especially, there are differences in the shape of the eye pit (or differences between sunken eyes, clear-cut eyes etc.) depending on the racial difference between the Orientals and the Westerners, the difference in stages of growth between adults and children or the sexual difference between men and women.
Accordingly, in the case that the eye cup is used in a state where it is pulled out from the eyepiece barrel, it is necessary for a person who observes with naked eye to finely adjust the position of the eye cup (by, for example, changing the distance L between the outer end of the eye cup and the iris I) to adjust the position of the iris I to coincide with the position of the eye point EP.
However, the eyepiece device disclosed in the above-mentioned Japanese Utility Model No. 3090513 suffers from the problem that when the eye cup is pulled out from the eyepiece barrel, fine position adjustment cannot be carried out in the range that requires finer adjustment, since the three pin engagement portions are formed at equal intervals.